This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Large-area vertical rutile TiO2 nanorod arrays (TNAs) were grown on F/SnO2 conductive glass using a hydrothermal method at low temperature. A self-powered ultraviolet
(UV) photodetector based on TiO2 nanorod/water solid–liquid heterojunction is designed and fabricated. These nanorods
offer an enlarged TiO2/water contact area and a direct pathway for electron transport simultaneously. By
connecting this UV photodetector to an ammeter, the intensity of UV light can be quantified
using the output short-circuit photocurrent without a power source. A photosensitivity
of 0.025 A/W and a quick response time were observed. At the same time, a high photosensitivity
in a wide range of wavelength was also demonstrated. This TNA/water UV detector can
be a particularly suitable candidate for practical applications for its high photosensitivity,
fast response, excellent spectral selectivity, uncomplicated low-cost fabrication
process, and environment-friendly feature.

Keywords:

TiO2; Nanorod; Ultraviolet photodetector; Solid–liquid heterojunction

Background

Ultraviolet (UV) photodetector has been a popular research issue for its potential
applications in a wide range of fields, such as remote control, chemical analysis,
water purification, flame detection, early missile plume detection, and secure space-to-space
communications [1]. To avoid the use of filters and achieve visible-blind operation, wide bandgap semiconductors,
such as GaN, SiC, ZnO, and TiO2[2-8], have been studied during the last decade for wide-spreading usage in photodetection,
especially in the ultraviolet region. Among conventional available UV photodetectors,
quite many kinds of structures have been fabricated, which in most cases are based
on epitaxial growth process and various solid-state junction structures. Typical examples
are photodetectors based on p-n junction, p-i-n photodiodes, Schottky barrier (SB),
metal–semiconductor-metal, and metal-insulator-semiconductor structures [9-15]. These photodetectors typically require an external bias as the driving force to
prevent the recombination of photogenerated electron–hole pairs. For large-area two-dimensional
arrays that contain huge amounts of small UV sensors, energy supply will be one of
the main challenges for such sensor systems.

Recently, self-powered nanodevices and nanosystems have attracted lots of attention
due to their various advantages. Xu et al. fabricated a nanowire pH sensor and a nanowire
UV sensor powered by a piezoelectric nanogenerator equipped with a capacitor, demonstrating
a self-powered system composed entirely of nanowires [16]. Yang et al. reported a self-powered ultraviolet photodetector based on a single
Sb-doped ZnO nanobelt bridging an ohmic contact and a Schottky contact, in which high
photoresponse sensitivity and short response time were observed [17]. Bai et al. reported a ZnO nanowire array ultraviolet photodetector with self-powered
properties, in which a high sensitivity of 475 without external bias is found [18]. Although n-type semiconducting ZnO is a significant material for optoelectronic
applications, it is unstable under both acidic and alkaline conditions. Also, the
photoresponse of ZnO-based UV detector is sensitive to the surrounding atmosphere
and can be easily affected by oxygen as well as water molecules. On the other hand,
TiO2 nanostructures have also emerged as very promising materials for optoelectronic devices
due to their excellent physical and chemical properties, such as high melting point,
chemical inertness, physical stability, direct bandgap (rutile 3.0 eV), high photoconversion
efficiency, and photostability. Self-powered UV photodetectors based on a photochemical
cell have been fabricated using a liquid I-/I3- redox couple electrolyte and a nanocrystalline TiO2 film [19] or a multilayer TiO2 nanorod-assembled cloth/nanorod array-based electrode [20]. Impressive performances were observed in these UV detectors. However, liquid I-/I3- redox couple electrolyte is not ideal for long-term operation: it is highly corrosive,
volatile, and photoreactive, interacting with common metallic components and sealing
materials. From this point, water-based electrolytes may be the safest, most stable,
and most environment-friendly electrolyte. Lee et al. reported a UV detector based
on TiO2/water solid–liquid heterojunction [21]. This self-powered UV photodetector behaves similar to a Schottky diode and works
in photovoltaic mode. Moreover, TiO2/water solid–liquid heterojunction UV detector exhibits high photosensitivity, excellent
spectral selectivity, linear variations in photocurrent, and fast response. Cao et
al. reported the photocurrent response of TiO2 nanorod arrays under UV illumination using a 0.5 M Na2SO4 aqueous electrolyte [22], in which TiO2 nanostructures can harvest more incident light photons compared to a flat thin-film
active layer because of the markedly enlarged TiO2/electrolyte contact area. However, they did not report its photosensitivity and spectral
response. All of these reported results indicate that self-powered UV detectors based
on TiO2 nanostructures show great potential as excellent candidates for commercial UV photodetectors.
Further advancements for TiO2-based self-powered UV detectors demand a deeper understanding of the main parameters
determining the photoelectric behavior, which also requires additional research and
insight into the electrical transporting process in these nanostructured devices.

Methods

Growth of TiO2 nanorod arrays by hydrothermal process

The single-crystalline rutile TNAs used for this study were grown vertically on FTO
glass using the following hydrothermal methods: a diluted hydrochloric solution was
prepared by mixing 50 mL of deionized water with 40 mL of concentrated hydrochloric
acid and was stirred at ambient temperature for 5 min, and then 400 μL of titanium
tetrachloride was added to the mixture. After being stirred for another 10 min, the
mixture was injected into a stainless steel autoclave with a Teflon container cartridge.
The FTO substrates were ultrasonically cleaned and were placed at an angle against
the Teflon container wall with the conducting side facing down. Hydrothermal synthesis
was conducted at 180°C for 2 h. After synthesis, the autoclave was cooled to room
temperature under flowing water, and the FTO substrates were taken out, rinsed thoroughly
with deionized water, and annealed at 500°C for 1 h to improve the crystalline structure.

Assemble of TNA/water solid–liquid heterojunction

The schematic structure of the TNA/water solid–liquid heterojunction UV photodetector
is shown in Figure 1. For device fabrication, the TNA layer grown on FTO glass was used as the active
photoanode. Pt counter electrodes were prepared by depositing a 20-nm Pt film on FTO
glass using magnetron sputtering. A 60-μm-thick sealing material (SX-1170-60, Solaronix
SA, Aubonne, Switzerland) was pasted onto the Pt counter electrodes. Afterward, the
Pt counter electrode and a nanostructure TNA photoanode were sandwiched and sealed
with the conductive sides facing inward. Finally, some high-quality deionized water
was injected into the space between TNA/FTO glass and Pt/FTO glass electrodes as an
electrolyte. A solid–liquid heterojunction UV photodetector was then fabricated, and
the active area of the TNA/water device for UV light detection was about 0.126 cm2.

Characterization of the TNA samples and the UV photodetector

The crystal structure of the TNA samples were examined by X-ray diffraction (XRD;
XD-3, PG Instruments Ltd., Beijing, China) with Cu Kα radiation (λ = 0.154 nm) at a scan rate of 2°/min. The surface morphology was characterized using
a scanning electron microscope (SEM; Hitachi S-4800, Hitachi, Ltd., Chiyoda, Tokyo,
Japan). The optical transmittance was measured using a UV-visible dual-beam spectrophotometer
(TU-1900, PG Instruments, Ltd.). The photoresponse characteristics of the self-powered
UV detector in the dark and under illumination were recorded with a programmable voltage–current
source (2400, Keithley Instruments Inc., Cleveland, OH, USA). A 500-W xenon lamp (7ILX500,
7Star Optical Instruments Co., Beijing, China) equipped with a monochromator (7ISW30,
7Star Optical Instruments Co.) was used as light source for spectral response characterization.
For the photoresponse switching behavior measurement, a UV LED (NCSU033B(T), Nichia
Co., Japan) with a wavelength of 365 nm was used as light source, and the photocurrent
was obtained by an electrochemical workstation (RST5200, Zhengzhou Shirusi Instrument
Technology Co. Ltd, Zhengzhou, China).

Results and discussion

The well-aligned TNAs with pure rutile phase are verified by the XRD pattern in Figure 2a. The θ-2θ scan pattern shows that the TiO2 nanorods grown on FTO-coated glass substrates have a tetragonal rutile structure
(JCPDS 02–0494). The SnO2 peaks are due to the pattern of FTO glass substrate. The reason that the hydrothermal
growth method delivers rutile phase instead of other phases, such as anatase and brookite,
could be attributed to the small lattice mismatch between FTO and rutile. Both rutile
and SnO2 have near-identical lattice parameters with a = 4.594, c = 2.958 Å and a = 4.737, c = 3.185 Å for TiO2 and SnO2, respectively, making the epitaxial growth of rutile TiO2 on FTO film possible. On the other hand, anatase and brookite have lattice parameters
of a = 3.784, c = 9.514 Å and a = 5.455, c = 5.142 Å, respectively. The production of these phases is unfavorable due to a very
high activation energy barrier which cannot be overcome at the low temperatures used
in this hydrothermal reaction. Figure 2b,c shows the micrographs of an as-grown TiO2 nanorod array taken by a field emission scanning electron microscope at tilted and
top views. The images at different magnifications and at different locations reveal
that the entire surface of the FTO-coated glass substrate is uniformly covered with
ordered TiO2 nanorods. Further analysis indicates that the nanorods are typically 100 to 150 nm
in diameter and are tetragonal in shape with square top facets consisting of many
small grids. The density of nanorods is typically 20 nanorods/μm2. No significant changes in nanorod array morphology were observed after annealing
at 500°C.

The optical property of the TNA was investigated using UV-visible transmittance spectrum.
Figure 3 shows the optical transmittance spectra of the TNA sample and the FTO glass substrate.
An obvious sharp absorption edge can be observed at 420 nm, which can be attributed
to the energy bandgap of rutile TiO2 nanorods. As the size of the TiO2 nanorod is well above the TiO2 Bohr exciton diameter, no obvious blueshift caused by quantum confinement is observed.
The low transmittance (20% to 30%) in the wavelength ranges of 400 to 550 nm is caused
by the strong light scattering from TNAs. An absorption edge for the FTO glass substrate
is about 310 nm, as shown in the inset of Figure 3. From these two transmittance spectra, we can conclude that only light with the wavelength
between 310 and 420 nm can reach the TNAs and contribute to the UV photoresponsivity,
which is confirmed in the following spectral response characterization.

Typical current–voltage (I-V) characteristics of the UV detector are shown in Figure 4. An SB-like behavior of the UV detector is demonstrated from the dark I-V curve, which shows a forward turn-on voltage of about 0.4 V and a rectification ratio
of about 44 at ± 0.6 V. Under the illumination of 1.25 mW/cm2 of UV light (λ = 365 nm), the UV detector shows an excellent photovoltaic performance, yielding
a short-circuit current of 4.67 μA and an open-circuit voltage of 0.408 V. This inherent
built-in potential arises from the SB-like TiO2-water interface, acts as a driving force to separate the photogenerated electron–hole
pairs, and produces the photocurrent. Therefore, this device can operate not only
at photodiode mode but also at photovoltaic mode without any external bias. The real-time
photocurrent response of the self-powered UV detector was measured at 0-V bias under
a 365-nm UV LED on/off switching irritation with an on/off internal of 5 s. Five repeat
cycles under an on/off light intensity of 1.25 mW/cm2 are displayed in Figure 5a, in which the photocurrent was observed to be consistent and repeatable. A fast
photoresponse can be clearly seen. From enlarged rising and decaying edges of the
photocurrent response shown in Figure 5b,c, the rise time and the decay time of the UV detector are approximately 0.15 and
0.05 s, indicating a rapid photoresponse characteristic. On the contrary, TiO2 one-dimensional UV photodetectors based on photoconductivity exhibit a much longer
recovery time due to the presence of a carrier depletion layer at the nanomaterial
surface caused by surface trap states [23]. The photosensitivity of the TNA self-powered UV detector to 365 nm light was also
tested using a range of intensities from 12.5 μW/cm2 to 1.25 mW/cm2. A steadily increasing photocurrent response was observed in relation to increasing
incident light intensity (not included here). This UV detector exhibits an excellent
capacity to detect very weak optical signals. Even under a weak incident light intensity
of 12.5 μW/cm2, the magnitude of photosensitivity has already approached two orders.

The wavelength selective ability of the TNA/water UV detector was measured in the
range of 260 to 550 nm at 0-V bias, and the result is shown in Figure 6. It is clearly seen that excellent UV light detection selectivity in a spectral range
between 310 and 420 nm is observed, which indicates that the device can be used as
photodetector for UV-A range (320 ~ 400 nm) application. The maximum responsivity
of the spectrum is about 0.025 A/W, located at the wavelength of 350 nm. The spectral
response edge of 310 nm is limited by the transmittance of the FTO glass substrate.
The edge of 420 nm is attributed to the absorption edge of the TNA layer.

The working principle of the device is discussed simply in the following. When UV
light (310 ~ 420 nm) shines on the TNA/water UV detector, the incident photons that
pass through the FTO glass into the TNAs and electrons in TiO2 are excited from the valence band to the conduction band and then generate electron–hole
pairs in the TNAs. The built-in potential produced by solid–liquid heterojunction
separates the UV light-generated electron–hole pairs. The separated holes move from
the valence band of the TNAs into the interface of TNA/water, subsequently seizing
the electrons from the water OH- anions (h+ + OH- → HO·). Considering the quite large TNA/water surface area, the small diameter of
the nanorods, and the built-in interface potential, a fast removal of holes from the
surface can be expected. On other hand, the separated electrons transport into the
TNA conduction band and are collected easily by the FTO contact as the work function
of FTO matches the conduction band of TiO2. These electrons move into the external circuit and then come back to the Pt layer
of the detector, thereupon returning the electrons to HO· radicals (e- + HO· → OH-) at the interface of water/Pt. In this way, the built-in potential makes the UV detector
generate photocurrent without any external bias. Even though zero bias is applied,
the UV detector exhibits high photosensitivity [21,24].

Conclusions

In conclusion, a photoelectrochemical cell-structured self-powered UV photodetector
was developed using water as the electrolyte and a rutile TiO2 nanorod array as the active photoelectrode. This device exhibits a prominent performance
for UV light detection. Under ambient environment, the photocurrent responses rapidly
with UV light on/off switching irradiation. Also, this self-powered TNA/water UV detector
demonstrates high photosensitivity and excellent spectral selectivity. All of these
results indicate that this novel UV detector can be a promising candidate as a low-cost
UV photodetector for commercially integrated photoelectronic applications.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

The work presented here was performed through the collaboration of all authors. YX
carried out the measurements of the TNA/water UV detector and drafted the manuscript.
LW conducted the transmittance spectra measurements. GW grew the TNA photoanode. QL
carried out the XRD and the SEM characterizations. DW deposited the Pt film and helped
fabricate the device. YC supervised the work and finalized the manuscript. SY and
GL analyzed the results and participated in the revision of the manuscript. LM and
JJ proofread the manuscript and corrected the English. All authors read and approved
the final manuscript.

Acknowledgments

This work was supported by the National Key Basic Research Program of China (2013CB922303,
2010CB833103), the National Natural Science Foundation of China (60976073, 11274201,
51231007), the 111 Project (B13029), and the Foundation for Outstanding Young Scientist
in Shandong Province (BS2010CL036).